1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Description of the Related Art
In electrophotographic image forming apparatuses, residual toner remaining on the surface of a photoreceptor even after a toner image thereon is transferred onto a recording material or an intermediate transfer medium is removed therefrom using a cleaner.
Strip-shaped cleaning blades made of an elastic material such as polyurethane rubbers are typically used for such a cleaner because of having advantages such that the cleaner has simplified structure and good cleanability. Among such cleaning blades, a cleaning blade in which one end thereof is supported by a supporter, and an edge of the other end is contacted with a surface of a photoreceptor to block and scrape off residual toner on the photoreceptor, thereby removing the residual toner from the surface of the photoreceptor.
In attempting to meet a recent need of forming high quality images, there are image forming apparatuses using spherical toner (hereinafter referred to as polymerization toner), which has a relatively small particle diameter and which is prepared by a method such as polymerization methods. Since such polymerization toner has such an advantage as to have a higher transfer efficiency than pulverization toner, which has been conventionally used, the polymerization toner can meet the need. However, polymerization toner has such a drawback as not to be easily removed from a photoreceptor by a cleaning blade. This is because such polymerization toner has a spherical form and a small particle diameter, and easily passes through a small gap between the tip of a cleaning blade and the surface of a photoreceptor.
In attempting to prevent polymerization toner from passing through a gap between a cleaning blade and a photoreceptor, it is necessary to increase the pressure to the cleaning blade contacted with the surface of the photoreceptor to enhance the cleanability of the cleaning blade. However, when the contact pressure of the cleaning blade is increased, the friction between the cleaning blade and the photoreceptor is increased, and thereby the tip of the cleaning blade is pulled by the photoreceptor in the moving direction of the photoreceptor. Specifically, as illustrated in FIG. 8(a), a cleaning blade 62 is pulled by the surface of an photoreceptor 123 in a moving direction (indicated by an arrow) of the photoreceptor due to increase of friction between the blade and the photoreceptor, thereby causing a problem (hereinafter referred to as everted-tip problem) in that an edge 62c of a tip 62a of the blade 62 is everted. In this regard, the thus everted tip has a restoring force, and therefore the tip tends to vibrate, resulting in generation of fluttering sounds.
In addition, when the cleaning operation is continued while the edge 62c of the cleaning blade 62 is everted, a portion of the tip 62a of the cleaning blade 62, which is apart from the edge 62c by few micrometers, is abraded as illustrated in FIG. 8(b). When the cleaning blade 62 is further used for the cleaning operation, the portion of the tip 62a of the blade 62 is further abraded, resulting in lack of the edge 62c of the blade 62 as illustrated in FIG. 8(c). The cleaning blade 62 having no edge cannot remove residual toner from the surface of the photoreceptor 123, thereby forming an abnormal image in which background thereof is soiled with residual toner.
In attempting to prevent occurrence of the problem, Japanese Patent No. JP-3602898-B1 (Japanese published unexamined application No. JP-H09-127846-A) discloses a cover layer made of a resin, which is harder than a rubber and has a pencil hardness of from B to 6H, is formed at least on the edge of the tip of a cleaning blade made of a polyurethane elastomer. It is described therein that by forming such a cover layer, friction between the tip of the cleaning blade and a surface of a photoreceptor can be reduced while enhancing the abrasion resistance of the cleaning blade. In addition, it is described therein that since the cover layer is hard and is not easily deformed, occurrence of the everted-tip problem can be prevented.
Further, in attempting to prevent occurrence of the everted-tip problem, Japanese published unexamined application No. JP-2004-233818-A discloses a blade impregnated with an ultraviolet crosslinkable material including a silicone so as to be swelled is exposed to ultraviolet rays so that the surface of the blade is covered with the crosslinked material, which is harder than rubbers and has good abrasion resistance. In addition, Japanese published unexamined application No. JP-2011-138110-A discloses a cleaning blade formed by impregnating the surface thereof with an acrylate polymer at a depth of from 5 to 100 μm, layering the acrylate polymer thereon and irradiating the acrylate polymer with an ultraviolet to be crosslinked. It is described therein that by covering the surface of the cleaning blade with an ultraviolet crosslinked material having higher hardness than the elastic blade, the abrasion resistance of the cleaning blade can be enhanced. In addition, it is described therein that since the surface portion of the blade is made of a combination of a rubber and a crosslinked material and has higher hardness than a rubber, the hard portion of the blade can be normally contacted with a photoreceptor even when the blade is used over a long period of time and the cover layer of the blade is abraded, resulting in prevention of serious abrasion of the blade and generation of fluttering sounds.
However, even when the above-mentioned cleaning blades are used, occurrence of the above-mentioned problems is hardly prevented if images having a high image area proportion (such as image having large solid images) are continuously produced (i.e., if the amount of residual toner on a photoreceptor to be removed by the cleaning blade is large). The reason is considered to be as follows.
Specifically, since the blade has a cover layer on the tip thereof or includes a crosslinked material in a surface portion thereof in the longitudinal direction thereof, the elastic property of the rubber of the blade tends to deteriorate. When the elastic property of the blade is deteriorated, the blade cannot be satisfactorily contacted with the surface of a photoreceptor (i.e., the pressure of the blade to a photoreceptor varies) if the photoreceptor is eccentric or the surface thereof is waved. In addition, when images having high image area proportions are continuously produced and a large amount of residual toner is present on the surface of the photoreceptor, the large amount of toner is collected at the tip of the blade by being blocked by the blade. In this case, the residual toner at the tip of the blade tends to pass through a relatively large gap formed between a portion of the blade and the surface of the photoreceptor, which are contacted with each other at a relatively low pressure due to eccentricity of the photoreceptor or waving of the surface thereof, resulting in occurrence of the above-mentioned abnormal image problem.
In the cleaning blade disclosed in Japanese Patent No. JP-3602898-B1 (Japanese published unexamined application No. JP-H09-127846-A), when a thick cover layer made of a material having high hardness is formed, the elastic property of the rubber constituting the blade is deteriorated by the rigid cover layer, and thereby the blade cannot be satisfactorily contacted with the surface of a photoreceptor (i.e., the pressure of the blade to a photoreceptor varies). Therefore, the cover layer is preferably as thin as possible. However, when a thin cover layer is formed, the cover layer is easily worn out and the rubber is exposed after a short period of time, resulting in occurrence of the above-mentioned everted-tip problems.
In addition, the cleaning blades disclosed in Japanese published unexamined applications Nos. JP-2004-233838-A and JP-2011-138110-A have the following drawback. Specifically, in order to impart the same hardness as that of the cover layer of the first mentioned cleaning blade to the surface of the crosslinked material-impregnated cleaning blade, it is necessary that the blade is impregnated with a large amount of ultraviolet crosslinkable material to such an extent that the surface of the blade is covered with the crosslinkable material. In this case, the ultraviolet crosslinkable material penetrates into an inner portion of the blade. When ultraviolet rays irradiate the blade to crosslink the ultraviolet crosslinkable material, the material-impregnated portion of the blade, which has a considerable thickness, becomes too hard, thereby deteriorating the elasticity of the rubber (blade), resulting in occurrence of the above-mentioned abnormal problem. In contrast, when the blade is impregnated with a small amount of ultraviolet crosslinkable material to maintain the elasticity of the blade, the rubber of the blade is not perfectly covered with the crosslinked material (i.e., the surface portion is constituted of a combination of the rubber and the crosslinked material), and therefore the surface portion has lower hardness than the cover layer of the first-mentioned blade. In addition, friction between the surface of the blade and a surface of a photoreceptor cannot be satisfactorily decreased, thereby causing the above-mentioned everted-tip problem. Further, the photoreceptor has a surface layer abrasion problem and the fluttering sound problem.
As a method of preventing the surface layer abrasion and the fluttering sound of a photoreceptor, a combination with a photoreceptor having improved durability is available. As a method of increasing the abrasion resistance of a photoreceptor, methods of forming a crosslinked resin on the surface thereof are disclosed in Japanese published unexamined applications Nos. JP-H01-205171-A, JP-H07-333881-A, JP-H08-15887-A, JP-H08-123053-A, JP-H08-146641-A and JP-2011-145457-A, and Japanese Patents Nos. JP-3734735-B1 and JP-4443837-B1 (Japanese published unexamined applications Nos. JP-2002-341571-A and JP-2004-233881-A).
A combination of a cleaning blade coated with an UV curable resin and a durability-improved photoreceptor using a crosslinkable charge transport material in its surface layer is disclosed in Japanese published unexamined application No. JP-2010-191378-A.
This prevents initial abrasion of the cleaning blade and the surface of the photoreceptor, and the fluttering sound. However, only mechanical durability thereof is improved and cleanability required in recent electrophotographic process is not sufficiently improved. Particularly when they are used for long periods, a microscopic gap is formed therebetween because of a slight abrasion therebetween, and a toner scrapes through the gap, resulting in production of abnormal images.
Because of these reasons, a need exist for a cleaning blade which has good abrasion resistance and which can be satisfactorily contacted with a surface of a photoreceptor at substantially a constant pressure to satisfactorily perform a cleaning operation without causing the fluttering sound problem and the everted-tip problem.